The present invention relates to a control system for a lock-up clutch in a torque converter.
Usually, torque conveters are poor in power transmission efficiency owing to a slip between an input element and an output element because the power transmission is carried out such that a working fluid moved by the input element, such as a pump impeller, moves the output element, such as a turbine runner although a torque multiplication and a torque variation absorption are available.
Thus, a so-called lock-up type torque converter has been put into practice wherein an output element is mechanically connected to the input element during an operating condition of a power source where a torque multiplication and a torque variation absorption are not needed to eliminate a slip mentioned above for increased power transmitting effeciency. In the lock-up type torque converter, a lock-up control is carried out such that during an operating condition of the power source where the torque multiplication and torque variation absorption are needed, a lock-up clutch is released to render the torque converter to operate in a converter state (a converter range), whereas during an operating condition where the above mentioned functions are not needed, the lock-up clutch is engaged to render the torque converter to operate in a lock-up state (a lock-up range). In addition to this basic control strategy, the lock-up clutch is released when the power source is idling or when the transmission is in neutral even if an operation in lock-up range is demanded. Thus, the lock-up control is such that the torque converter frequently switches between the torque converter state and the lock-up state.
The lock-up clutch is operable in response to a difference between a lock-up pressure to which one side thereof is exposed and a converter pressure to which an opposite side thereof is exposed such that the lock-up clutch is released when the lock-up pressure is substantially equal to the converter pressure and is engaged when the lock-up pressure is discharged.
The lock-up torque converter of the above mentioned type poses a problem that substantial shocks are produced when the lock-up clutch is engaged if rate of engagement of the lock-up clutch is very high. The result is that the rate of reduction of the lock-up pressure which is to take place upon shifting from the converter state to the lock-up state is very quick. In order to alleviate this problem, it has been the common practice to provide an orifice so as to effect gradual reduction of the lock-up pressure. However, the provision of the orifice is not satisfactory in eliminating the substantial shocks.
Another control strategy of a lock-up clutch of a torque converter is such that during an operating condition of a power source where torque multiplication is not needed although there is a torque variation, a slip control is carried out such that a relative rotation between an input element and an output element is adjusted to a required minimum limit (a preset value) in order to enhance a power transmission effeciency while securing a slip in the torque converter necessary to absorb a torque variation.
The slip control thus far proposed, however, has not been satisfactory.